European Patent Application EP0788914-A3 published by European Patent Office in 1998 discloses a vehicle drive system which inputs an output torque of one or both of an engine and a motor/generator through a continuously variable transmission. This is the so-called parallel hybrid vehicle drive system.
The generator/motor in this system is driven as a motor when electrical force is supplied from the battery and outputs rotational torque to the drive wheels. On the other hand, when rotational torque is input from the drive wheels, it functions as a generator and charges the battery by performing so-called regenerative braking. The drive ratio of the continuously variable transmission, i.e., the ratio of the input and output rotation speeds of the transmission, varies depending on the vehicle speed and the throttle opening of the engine.